Friction stir welded joints which are in components have a virtually optimum joint structure in the bead formation area, which is virtually comparable to the original material characteristics of the components. The use of friction stir welding (FSW) for joining work pieces therefore makes it possible to produce weld beads which can be subjected to heavy loads and whose mechanical characteristics virtually correspond to those of the basic material. Known embodiments of apparatuses for friction stir welding have a friction stir welding head with a welding pin which rotates about its longitudinal axis and is coaxially surrounded by a shoulder tool which rotates in synchronism with the welding pin. Together with the shoulder tool, the welding pin forms the actual welding tool. In order to join two work pieces, for example in the form of two aluminum sheets, the rotating welding tool, comprising the welding pin and the shoulder tool, is moved along and through the abutment area between the two work pieces. The material of the two work pieces becomes plastic in the bead formation area as a result of the friction heat produced by the rotation of the welding tool. At the same time, the rotating welding pin mixes the material of the mutually abutting work pieces in the bead area.
The shoulder tool which rotates in synchronism with the welding pin is in this case moved parallel to the work piece upper face, and ensures that the bead has a flat surface.
In known apparatuses, the friction stir welding head can frequently be positioned with high accuracy, controlled by a CNC open-loop control system, with respect to the work pieces by means of a gantry arrangement, on at least three spatial axes.
The work pieces rest on a stable work piece rest. A narrow strip of metal (backing) is incorporated underneath the bead formation area in the work piece rest, and can quickly be replaced when it becomes worn. The work pieces are secured on the work piece rest by clamping apparatuses, in order to prevent the work pieces from being moved by the high mechanical forces which occur during the friction stir welding process.
The distance between the welding pin or the welding pin tip and the work piece rest, or the backing incorporated in it, is of critical importance to the quality of a weld bead produced using the friction stir welding method. If the distance is too short, this can lead to increased wear or failure of the welding pin. In contrast, if the distance is too long, the material will not be stirred completely by the welding pin, so that the components are not completely welded to one another in the rearward area of the weld bead, and the weld bead can break when subjected to a bending load. This results in a so-called “lack of penetration” (LOP) fault.
It is therefore of major importance for the achievable quality of a weld bead formed using the friction stir welding method to know as accurately as possible the vertical position of the welding head, at least with respect to the work piece rest or the backing (z-axis). Known gantry apparatuses for friction stir welding have a specific position measurement sensor for detection of the z-position of the welding head in space. However, since this measurement is carried out at a short distance from the welding area, it is not possible, for example, to determine heat-dependent length changes of the welding pin or bending of the gantry arrangement, for guidance of the welding head.